Estrogen hormone action in target tissues of the body involves the interaction of the hormonal substance with a receptor protein. The specificity and responsiveness of tissues to hormonal stimulation are governed in most part by the presence and biochemical action of this receptor protein. Nuclear localization of the receptor protein and its activity was shown to be increased by exposure to growth factors, suggesting a coupling of these two signaling pathways. Investigations of the action of various antiestrogens determined that ICI-164384 is a pure antagonist functioning through a mechanism resulting in a rapid loss of the receptor protein within target tissues. This is a unique mechanism not previously described for other hormonal antagonists. Estrogen has an associated effect on bone homeostasis which has now been shown to occur through a receptor mediated mechanism. Bone cells have extremely low estrogen receptor levels. Two experimental bone cell lines were stably transfected with the estrogen receptor. Estrogen was shown to specifically stimulate exogenous and endogenous genes in the clones and parietal cell lines. Prenatal or neonatal estrogen exposure of mice has shown for the first time a highly sensitive and significant influence on adult bone density. Estrogen receptor is being expressed in yeast for analysis of the gene transcriptional activity of the protein using a variety of hormonally regulated promoter sequences linked to beta-galactosidase reporter genes. Site directed mutagenesis of the ligand binding domain of the receptor protein has identified specific amino acids which influence transactivation activity with no effect on ligand binding. Other mutants are being tested to identify a stereochemical ligand recognition site in the protein which biochemical analysis has shown is critical for ligand affinity binding and agonist activity.